The dewaxing of hydrocarbons to liquids of lower pour point is a process of great commercial significance. Although alternatives exist, it is now evident that the use of shape-selective catalysts, such as the intermediate pore size zeolite catalysts, to selectively convert those paraffins that contribute the most to high pour points has many advantages over other methods. Thus, catalytic dewaxing over shape-selective zeolites will likely be the most commercially significant dewaxing process in the hydrocarbon processing industry.
Catalytic dewaxing of hydrocarbon oils to reduce the temperature at which precipitation of waxy hydrocarbons occurs is a known process and is described, for example, in the Oil and Gas Journal, Jan. 6, 1975, pages 69-73. A number of patents have also described catalytic dewaxing processes. For example, U.S. Pat. No. Re. 28,398 describes a process for catalytic dewaxing with a catalyst comprising a zeolite of the ZSM-5 type and a hydrogenation/dehydrogenation component. U.S. Pat. No. 3,956,102 describes a process for hydrodewaxing a gas oil with a ZSM-5 type catalyst. U.S. Pat. No. 4,100,056 describes a mordenite catalyst containing a Group VI or Group VIII metal may be used to dewax a distillate from a waxy crude. U.S. Pat. No. 3,755,138 describes a process for mild solvent dewaxing to remove high quality wax from a lube stock, which is then catalytically dewaxed to specification pour point.
Catalytic dewaxing processes may be followed by other processing steps such as hydrodesulfurization and denitrogenation in order to improve the qualities of the product. For example, U.S. Pat. No. 3,668,113 describes a catalytic dewaxing process employing a mordenite dewaxing catalyst which is followed by a catalytic hydrodesulfurization step over an alumina-based catalyst. U.S. Pat. No. 4,400,265 describes a catalytic dewaxing/hydrodewaxing process using a ZSM-5 type catalyst wherein gas oil is catalytically dewaxed followed by hydrodesulfurization in a cascade system.
In catalytic dewaxing processes using shape-selective catalysts, such as the intermediate pore size catalysts such as ZSM-5, the waxy components, particularly the n-paraffins, are cracked by the zeolite into lighter products containing olefinic material. However, these lighter products have a boiling point (b.p.) predominantly 610.degree. F..sup.-. Olefinic fragments are unstable to oxidation; so the dewaxed oil may be subsequently hydrogenated over catalysts to saturate the olefins and improve the oxidation stability of the oil. The hydrogenation catalysts generally used are mild hydrogenation catalysts, such as a CoMo/Al.sub.2 O.sub.3 type. The color of the oil may also be improved in this hydrofinishing process.
Recent experience with ZSM-5 based catalytic dewaxing has shown that the dewaxing of higher boiling point or residual feeds (greater than 610.degree. F. b.p.) pose significantly greater problems than that experienced with lighter distillate feeds. Accelerated catalyst aging is particularly a problem for the heavier feeds which cause catalysts to display a more rapid loss of activity relative to a fixed pour point specification (e.g. -7.degree. C.). This loss of activity results in higher aging rates (so that the operating temperature of the dewaxing catalyst must be increased to maintain desired conversion) and lower on-stream capacity factors.
It would be desirable to find a way to prolong hydrodewaxing catalyst activity at low cost.
In copending patent application Ser. No. 685,564, filed Dec. 24, 1984, U.S. Pat. No. 4,648,957 on Mar. 10, 1987) there is described a method for reducing catalyst aging rate by separation of olefins during the dewaxing process. The entire disclosure of Ser. No. 685,564 (U.S. Pat. No. 4,648,957) is incorporated in this specification by reference for a disclosure of that process and its utility with a process for converting the separated olefins to higher boiling products including gasoline, middle distillates and lubricants. The process described in Ser. No. 685,564 (U.S. Pat. No. 4,648,957) was based upon the finding that olefinic intermediates (primary by-products) which are by-products of the catalytic hydrodewaxing reaction which occur predominantly in the distillate and lighter fractions in sizeable quantity, significantly inhibit the dewaxing reactions and, in addition can cause an uncontrolled reactor behavior such as temperature runaway. The process of Ser. No. 685,564, U.S. Pat. No. 4,648,957, removes the olefinic intermediate products as they are formed during the course of the hydrodewaxing reaction so as to simultaneously ensure a controlled reactor behavior. The enhanced activity gives a lower start-of-cycle temperature and a larger operating temperature span, thereby resulting in longer catalyst cycle life.
The olefinic intermediates may be removed as they are formed during the course of the hydrodewaxing reaction and sent through an olefin conversion unit e.g. an MOGDL unit to catalytically oligomerize them into additional low pour point high quality lube. Combining catalytic hydrodewaxing with MOGDL simultaneously enhances the hydrodewaxing catalyst activity, increases the lube yield and ensures a controlled reactor behavior. Furthermore, blending of the low pour MOGDL lube into the main lube product stream reduces the requirement of hydrodewaxing severity to meet a given target pour point.
Co-pending application Ser. No. 816,092, filed Jan. 3, 1986, discloses a process in which the olefins are separated from the partly dewaxed effluent and hydrogen injected. Although that process obviates the problems associated with the presence of the olefins, it requires recycle of hydrogen to an intermediate point in the process which adds to equipment cost and complexing as well as to operational complication. It would be desirable, therefore, to find a simpler way of dealing with the problems arising from the presence of the olefins in the dewaxing stream.